This invention relates to an optical filter for sensing and detecting laser radiation. More particularly, this invention relates to a laser discrimination device which is based on temporal coherence and is not affected by atmospheric turbulence.
In many applications in which lasers are used, sensing, filtering and protection from laser radiation is required. Under certain restricted conditions, features such as wavelength, energy, power, pulse width, repetition rate and polarization, or a combination of those may be utilized to discriminate laser emissions from the background radiation. However, a feature that is truly unique to all lasers is the coherence properties of the radiation field. The light from a laser is both spatially and temporally coherent while that from thermal sources is not. Therefore, for the purposes of laser sensing, filtering and protection, it is highly desirable to have devices that discriminate lasers based on the coherence properties of the incident light. For example, such a device can be used for detecting enemy lasers with unknown frequencies or variable frequencies, e.g. frequency agile lasers.
It is known that the spatial coherence properties of both the laser radiation and the ambient radiation (in the following, the term "ambient light" refers to non-laser light) can change significantly through propagation. The degree of spatial coherence of a laser beam can be dramatically reduced by turbulent atmosphere. Furthermore, it is also known that the light from completely unrelated emitting atoms (molecules) can become spatially coherent if the propagation distance is large. This fact implies that it may not be possible to design a laser discriminating device based on the spatial coherence of the incident light. However, the temporal coherence of lasers is not distorted by natural propagation effects. Therefore, devices relying on the detection of the temporal coherence of the incident light are most desirable. In the past, several devices that use the temporal coherence of the incident light have been investigated. They were all based on the Fabry-Perot interferometers.
A Fabry-Perot etalon interferometer consists normally of two plane, parallel partially reflecting surfaces formed on a solid glass spacer so that one portion of incident radiation is transmitted directly through while other portions, being reflected between the partially reflecting surfaces before emerging, are transmitted over a longer path. Examples of laser sensing devices based on Fabry-Perot etalon are described in U.S. Pat. Nos. 3,824,018, 4,536,081, 4,536,089 and 4,600,307.
Unfortunately, the applications and performance of the known etalon devices are limited by the intrinsic design of the Fabry-Perot interferometer. Accordingly, there is a need for improved laser sensing devices whose performance are based on temporal coherence.